Method of grinding



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March 2 March 28, 1939. v L DE Re. 21,039

METHOD OF GRINDING Original Filed Oct. 14, 1932 3 SheetS -Sheefi 2 [litIWIHII INVENTOR JAMES M SLADE we Y ATTORNEY March 28, 1939. .1. v. SLADEMETHOD OF GRINDING Original Filed Oct. 14, 1932 5 Shets-Sheet 3 TOTH/CKENER INVENTOR.

JAMES KSIADE BY (Lab; (km

ATTORNEY Reissued Mar. 28, 1939 METHOD OF GRINDING James V. Slade, NewRochelle, N. Y., asslgnor to The Don- Company, Inc., New York, N. Y., acorporation of Delaware Original No. 2,052,426, dated August 25, 1938,

Serial No. 637,696, October 14, 1932.

Reissue No. 20,941, dated December 6, 1938, Serial No. 226,039, August23, 1938, This application for reissue February 1, 1939, Serial No.254,071

16 Claims. (Cl. 83-.04)

This invention relates to improvements in grinding systems, moreparticularly the two stage closed circuit system which is more or lessstandard in the metallurgical industry and also used in the non-metallicmineral industry, for instance in the cement industry.

One object of this invention is to provide greater flexibility ofoperation in a closed circuit two stage grinding system. Morespecifically this means rendering more positive and flexible, theoperation of the primary classifier, which heretofore has labored undercertain shortcomings.

Other objects are to simplify the plant layout and to eliminate certainheretofore bothersome restrictions in the way of mechanicalarrangements, and to increase the degree of efiiciency and utilizationof existing machinery.

These and other advantages are obtained by a change in the fiow sheet orhook up, featuring essentially the return of part or all of thesecondary mill discharge to the primary classifier. The nature of thisarrangement will be better understood when it is recalled that astandard flow sheet for closed circuit grinding generally includes aprimary and a secondary closed grinding circuit with each circuitincluding a bill and an associated classifier. Both circuits operate inseries, with the overflow from the primary classifier feeding into thesecondary circuit or seondary classifier. The invention may'also be saidto consist in utilizing material obtained from one stage to control andimprove the operation of the preceding stage.

Standard equipment called for by this fiow sheet comprises a ball or rodmill for coarse grinding in the primary circuit, and another mill forfine grinding in the secondary circuit. For

' proper operation of the system the primary classifier must separateundersize particles up to a certain desirable size, which may be calledthe intermediate size. This is the size of material which is transferredtogether with finer sizes to the secondary cycle for fine grinding.

That intermediate size is not always readily or positively obtainable inthe ordinary run of rake classifiers owing to the fact that theseparation in the classifier depends upon a variety of factors such asrake speed, the amount of wash water used, the dilution of the pulp, andupon the area of the classifier bath from which the overflow orundersize is withdrawn. In other words the character of the separationdepends upon what in the pertinent terminology is called.

the overfiow' capacity" of the classifier, andwhich may be defined asthe tonnage of undersize solids up to a certain mesh size which themachine is capable of delivering free or substantially free fromoversize.

Since the above factors have bearing upon each other and upon theclassifier operation, they cannot be freely and individually changed forthe purpose of controlling the overflow capacity without imposing otherdifiiculties and limitations upon the classifier operation.

The operator tries to correct such discrepancies in various ways. Onetendency is to operate the classifier at increased speeds to try to keepthe bath better agitated to hasten its selective action which may leadto an otherwise undesirable excess of speed. Another tendency is:toreduce the amount of liquid passing through the classifier to reduce thedilution, which increases the buoyancy action upon the intermediate:size of solids, at which procedure he mayirun'rto a point at whichproper selectivity is lost.,--.-t;Moreover, while this reduction ofliquid contents produces a bath of excessive specific gravity andviscosity, it also imposes certain limitations upon the amount of spraywater allowable, which in turn results in a rake product of insuificientcleanliness.

The primary classifier is required to separate a desired undersize ofsolids in the overflow (intermediate plus finer sizes) -for furthertreatment in the secondary circuit, and further to have sufiicientraking capacity in order to handle the necessary amount of circulatingload. In other words the classifier must be selective in certain waysbut its selectivity must not be such that it makes its reaction feltupon other phases of the classifier operation. So there are twoantagonistic factors to be reconciled in this machine.

Still in other words the dlmculty is that the demand for a certainraking capacity results in a greater width of the classifier which inturn influences the overflow capacity", tending to produce a much finerseparation than desired for intermediate size.

Still another factor may tend to render difiicult the proper coarseseparation in the primary classifier, namely a certain minimum amount ofcirculating liquid is required for the proper operation of the ballmill, and this required minimum liquid under some circumstances exceedsthe amount desirable for the separation of the intermediate size in theclassifier overflow. .On the other hand this minimum amount of water mayhave to be augmented by extra water added to the mill feed if thedischarging rake product or sands from the classifier are too dry. Inthe latter case a considerable increase in rake speed is the only remedyif the desired separation of the intermediate size through theclassifier overflow is to be obtained,

Assuming the operator has succeeded in ad-' justing satisfactorily theoperation of the primary cycle under such diverse requirements, and noconsiderable changes in the feed size occur, the primary classifieroverflow enters the feed well of the secondary classifier, which ispreferably a bowl classifier. The difference in the function of the twoclassifiers should here be noted. The primary classifier is pressed intoa service for causing the separation of flnes plus certain solids of adesired intermediate size, with an attendant excess of "overflowcapacity". As this overflow enters the bowl of the secondary classifierit encounters a deliberately increased overfiow capacity in the form ofthe enlarged circular overflow launder of the bowl, which produces thevery fine separation desired. This overflow constitutes the finalproduct from this flow sheet. The material or intermediate size is rakedout by the classifier and circulated through the secondary grinder ortube mill to be reduced to said final size, and then to be separated outupon its return to the bowl of the secondary classifier. The finalslurry is usually dewatered in a thickener from which the sludge may befurther treated in a vacuum filter.

The connection between two such circuits may be said to be a bottle neckof the operation. This means that it is dimcult not only to adjust eachcircuit for proper and efllcient operation and cooperation of themachine units within itself, but it is also difiicult to tune upproperly the cooperation of the two circuits with each other.particularly in view of fluctuations in the amount and size of the freshfeed thereto. As a consequence of said fluctuations, one circuit may beoverloaded, as would appear from an excess of material loading down therakes, while the other circuit may operate below capacity. Thedifllculty therefore, is found in a certain lack of flexibility withinthe system, and which is further aggravated where each circuit, and theconnection between the circuits requires a pump for maintaining thetransit of the material.

Pumps in such operation may form a source of trouble in that they aresubject to heavy wear and tear due to the abrasive and coarse characterof the charge. Moreover, irregularities in their operation may causesudden surges which are liable to upset certain desirable quiescent fiowconditions, as for instance, in-the sensitive feed to the bowl of thesecondary classifier. Also the character of some materials may precludetheuse of pumps altogether.

Diificulties are furthermore encountered in the practical layout of suchplants, in that very carein] consideration must be given to the relativeelevations and locations of the various machine units to eflect theproper transit of the material. Various kinds oi mechanicalelevators andspecial feeders must frequently be resorted to in order to carry thematerial from one step of the operation to the next, when the dilutionof "the material and amount of coarse material therein, as in theprimary circuit, precludes the use of otherwise convenient and desirablepump arrangements.

main feature of this invention serving to overcome the difiicultiesenumerated, lies in the fact that in a suitable manner an auxiliaryvolume of slurry is made available from the process and introduced intothe bath of the primary classifier, so that it canbe said that theprimary classifier is operated with a tonnage in the overflow which isgreater than the tonnage of new feed entering the primary cycle.

The invention thus comprises the control of overflow capacity in theprimary classifier by the return of slime values thereto to aid coarserseparation in that step, but also the return if desirable of relativelycoarse material irom the secondary grinding step to the primary cyclefor the purpose of load regulationin the grindin stage. i

Hence the embodiment herein shown provides for returning or by-passingmaterial discharge from the secondary mill, through the primary1illassifier instead of direct to the secondary class- Furthermore, as aconsequence of the above changes of operation this invention makes theuse of pumps more generally possible and their operation positive,thereby displacing expensive elevators, and other bulky and unhandyarrangements.

Numerous advantages as hereinafter described are the result of theimproved arrangement according to this invention.

The manner of carrying out this invention may be understood by referringto the accompanying drawings in which Figure 1 is a diagram showing thepresent improvement over a standard system. Figure 2 is the sideelevation, systematicaliy shown of a practical arrangement of theimproved system. Figure 3 is a cross sectional view taken upon Figure 2,disclosing the comparatively large width of the primary classifier andthe distribution of the feed thereto. Figure 4 is a detail showing of amill unit including the primary and the secondary mill combined. Figure5 is a corresponding duplication of the showing of Figure 1, merelyillustrating the improvement in a somewhat dlfierent and moreconvenmatter is separated through the rake discharge while anintermediate size plus fines is discharged through the overflow. Theprimary rake product is returned to the primary feed, thus closing theprimary grinding cycle. The returning material constitutes the so calledcirculating load in closed circuit grinding.

The overflow i'romthe primary classifier contains intermediate and finesized material, which is transferred into the secondary grinding cycle,by way of feeding it into the secondary classifier which latteraccording to standard practice is the bowl type of classifier. In thesecondary classifier, the intermediate size is separated through therake discharge and removed, and after passing through the secondary millis circulated back into the bowl feed, thus closing the secondary cycle.The overflow from the bowl contains the fines which are delivered fromthe system as a final product indicated in the form of a thin pulp orslurry.

The improved arrangement of this invention is shown in diagram numberedFigure 1. Instead of tying the two cycles or circuits together with afeed line between the primary and the secondary classifiers it is to benoted that in Figure l the primary and the secondary circuits merge intoeach other as both mill discharges feed into the pool of the primaryclassifier, thus eliminating the usual bottle neck and attaining manyother advantages explained hereinafter.

In Figure 1, fresh feed 2! enters the primary mill 22. Upon leaving themill as indicated at 22 the material is separated in the primaryclassifier 23 where the coarse solids are caused to leave through therake discharge in order to re-enter the primary mill, thus closing theprimary circuit, while intermediate sized particles and'fines escape toenter the secondary circuit as they feed the secondary or bowlclassifier 24. There again separation takes place, with intermediatesized solids leaving the rake discharge towards the secondary mill 2!.The discharge from the secondary mill'as indicated at 28 joins theprimary mill discharge 22' in feeding the primary classifier 23. Finalfine size as overflows from the bowl leaves the secondary circuit asindicated at 21.

The arrangement in Figure 2 includes the mill unit 28 comprising aprimary and a secondary mill or compartments 29 and 30 respectively.Fresh feed 3| plus returned circulating load from a primary classifier32 enters a feed container 33 from which a customary scoop or spiralfeeder 34 may feed it into the primary grinding compartment of the unit.Discharge from the primary and from the secondary compartment join in acommon discharge chamber 33 between the compartments and leaving themill in the direction of the arrows through openings 38 in the drum todrop into a sump 31 below the mill from which a pump 33 lifts it throughriser 39 to the feed through 40 of the primary classifier 32. Theprimary classifier has a customary washing spray 4| for the rakeproduct, and is elevated substantially above the mill unit high enoughto allow the rake product to. gravitate at 42 to the primary mill feed,and the primary classifier overflow 43 to gravitate to feed a secondaryclassifier 44 which latter again is located high'enough to allow itsrake product to gravitate at 45 to the secondary mill feed. The overflowfrom the bowl 46 of the secondary classifier constitutes the finalproduct to be delivered from the grinding system, as indicated at 41.

An alternative manner of introducing the fresh feed is indicated in dotand dash at 48 where fresh feed may enter the feed trough 43 of theprimary classifier 32.

Figure 4 shows more clearly and in detail a preferred embodiment inwhich the primary and the secondary mill are combined in one rotary ballmill unit 43. The showing is intended to convey merely the feature thatseparate feeds enter at each end of the drum unit, and combined leave acommon central discharge chamber. The feeds enter for instance by way ofscoop members or feeders 53 and BI through hollow trunnions into aprimary and a secondary grinding compartment 52 and 53 respectively ofthe drum unit. The'ends of the drum are shown to be" supported inbearings 34 and 33. A gear 33 surrounding the circumference of the drumindicates the drive. The primary compartment is pool of the primaryclassifier forming the junction.

While the elimination of the bottle neck between the circuits is mostclearly illustrated in the largely diagrammatic showing of Figure l, a.more conventional showing of the flowsheets has been given in Figure 5.These figures correspond in substance exactly to the Figure 1, butafford a more accurate showing of the relative position of the machineswith respect to the location of the respective inlets and outletsthereof.

Figure 5 illustrates the improvement upon the standard flow-sheet,analogous to Figure l. The operation consists in feeding raw material atH to the primary mill 22, transferring the mill discharge 12 to theprimary rake classifier 23, and allowing the rake product to gravitateback as at 13 to feed the mill, while the classifier undersize 14 goesto feed the bowl of the secondary classifier 24. Rake product from thesecondary classifier feeds the secondary or fine grinding mill 25, mil]discharge 18 being returned over the pri-v mary classifier 23 to thesecondary classifier 24. Of course the transfer of material in thisflowsheet may be maintained by a single pump handling the combineddischarges from both mills to feed the primary classifier, in a mannerpreviously described in connection with Figures 2 and 3 (but here notshown). bowl overfiow at 11.

Fig. 6 shows a modification, or compromise between the fiowsheets inFigure 5 or 1 respec- Final size leaves the tively, in that it indicatesthat a portion of the material from the mill discharge is split off andreturned to the primary classifier or circuit. In other words, a portionof the material obtained from the secondary mill discharge is by-passedthrough the primary classifier or circuit, whereby it should beunderstood that the split off portion may be selected according to sizecharacteristics,

or quantity proportion, depending upon what the specific requirement ofoperation be. Figure 6 is distinguished substantially only by thearrange-' 'ment of a by-pass connection 18 between the secondary milldischarge and the primary classifier feed, branching of! a portion ofthe secondary load at a splitting point indicated at 13. Final sizeleaves the system through bowl overflow at 30. No particular mechanismis indicated at this point, while it is to be understood that the splitmay be effected in various suitable ways. More broadly, this alternativearrangement brings within the scope of this invention the idea that aselected portion of the material in the secondary circuit may bereturned to the primary classifier or primary circuit.

The practical advantages and consequences of this simple change ofarrangement, are explained asfollows: It was stated above that theexcess overflow capacity of the primary classifier is responsible forthe difiiculties encountered in obtaining the desired separation of acertain intermediate size. With the retum of the discharge from thesecondary or tube mill to the primary classifier, the difliculty ofseparation is largely relieved. This return of secondary mill dischargeconstitutes an auxiliary volume of slurry available to create thedesired overflow intensity in the primary classifier, and promotes thedesired separation of the desired size of solids in the primaryclassifier, in that it helps to lift, buoy up and flow away, the desiredintermediate sire of particles. This beneficial eflect, it will beunderstood, is due to the large proportion of line or slimes in thesecondary mill discharge. There is now produced a more intense flow overthe weir, without excessive dilution due to the presence of the slimes.

Expressing this in terms of buoyancy, the properties of added slimesproduces suflicient buoyant force to stimulate the separation. On theother hand, the amount of dilution may be balanced go against a moreliberal supply of spray or washwater in the classifier wherebyneitherthe thickness or viscosity of the bath nor the rake speed need beexcessive, and a clean rake product may be obtained. In other words, theslime values and added tonnage in the primary classifier permitrelatively higher dilutions for a given separation. Thus the operationof the primary classifier is rendered more flexible and free fromdisturbing factors. 1

While the present embodiment merely illustrates the return of materialfrom the secondary mill discharge to the primary classifier, it shouldbe understood that the added slime value in the primary classifier mayalso be produced from various other points or sources of the treatmentprocess. For instance it may be represented by a recirculated portion ofthe primary classifier itself, by the overflow from the secondaryclassiher, or else by the sludge recovered from a final,

4o thickening step past the secondary clarifler, in which alternativecases it can be said that all the undersize returned to the primaryclassifier is finer than the critical mesh of separation in the primaryclassifier.

' In the preferred embodiment of this invention,

the secondary mill discharge together with the primary mill discharge ispumped to feed the primary classifier. This afl'ords a three-foldadvantage. First the need for two circulating pumps (one for eachcircuit) in the old system, is reduced to only one, thereby reducing thefirst or installed cost and simplifying the plant layoutas well as itsoperation. This really means asaving of two pumps when considering thatnormally each pump requires a spare pump for such emergencies as wouldbe expected when handling material difllcult to pump and having abrasivequalities. Second, the lubricating efl'ect of the added slimes upon thecoarse sands obtained from the primary ball mill greatly improve themechanical operation and the service of the pump proper.-

This means less wear together with more reliable and smoother running ofthe pump. Moreover it my mean that only thus is the use of a pump 05made possible at all. Third, the irregularities of the pump operation,manifesting themselves normally in sudden surges or the like, are nowspread over'and absorbed in the entire system. The importance of thispoint will be evident upon comparison with the old arrangement whereindividual pumps serve the primary and the secondary circuits. A surgein the pump of the circuit would directly aflect the sensitive overflowof the bowl'of the secondary classifier, but with the improved one-pumparrangement, any surge ar,oso

therefrom is absorbed or dispersed through the bath or pool in theprimary classifier which is interposed between-the pump and the bowl.Furthermore, any possible surge is smoothed out through the otherwiseimproved operation of the a pump itself.

Due to the new arrangement as shown, the two closed circuits are nowmuch more intimately coupled with the effect that changes in the newfeed are more readily distributed over both cir- 10 cults, instead ofoverloading or choking one'circult, and leaving the other more or lessidle. Through the medium of the combined mill feed to the primaryclassifier, the efi'ects of such uneven loads are adjusted between thetwo circuits. To is be more specific corresponding to a change orfluctuation in the new feed size, the separation of the intermediatesize in the primary classifier can be varied between the practicallimits of say 0 and 66 mesh, whereby disregarding the original feed 20size, the load can be i ssled between the primary and the secondarycircuit so that both mills and both classifiers will be evenly taxed.The control of such separation can be readily efi'ected by a change inthe amount of spray or wash liquid in, as or of feed water to, theprimary classifier, or else by a change in the classifler'speed. In anycase, the primary classifier forming the coupling between the twocircuits will equalize or absorb the shocks from feed and otherfluctuations. an

' Thus can be obtained the desired even load distribution between theprimary and the secondary mill which is vital for their efflciency, andthe preservation of the grinding parts.

The flexibility in the system, giving opportunity 35 for even loaddistribution between the primary and the secondary mill oflers aspecific advantage in the shutting down operation of the mill. In caseof a shut-down for overhauling or repair, it is desirable to remove fromthe system a large 40 portion of the material normally in transit,before stopping operation of the plant. After stopping the feed, thesystem then has to be allowed to run the circulating load down to acertain minimum, which under ordinary conditions means 5 that the firstcircuit is depleted while the second is still operating under load.'Ihis involves the expenditure of considerable time for unproductivework, requiring sometimes an hour or more, and it is hazardous to runeither mill under a 50 greatly diminished load. Thanks to theflexibility afi'orded by the present improvement, it is now possible tomaintain the load evenly distributed over both circuits, as the loaddiminishes equally in each circuit. In this way, the system is freed 55from the load within a much shorter time because both mills are workingsimultaneously upon the dwindling load. Both circuits can be kept inbalance to exhaust the load in the classifier baths, which avoids havingthe primary mill run- 60 ning' empty while waiting for the secondarymill to finish up.

correspondingly, this arrangement also helps the starting up of thecircuits, by permitting coarser temporary separation to bring up thesecondary or tube mill load evenly or concurrently with the primaryload, instead of running very light until after the primary circuit hasbuilt up a heavy load. Thus, both circuits may start evenly and beloaded up quickly by tem- 7 porarily increasing the feed. Considerablesav .ings of time can thus be attained, and the wearandtearofgrindingpartsinthemillsbere duced.

Another practical consequence of great impor- 7s tance fiows from thisinvention in cases where the primary and secondary mills are coupled asa unit. In this unit, the primary mill is bound to the secondary millfor coaxal rotation, and it is therefore desirable, if not vital, thatboth mills shoulder be under a load while running. Heretofore, suchdifiiculty has been overcome only by discharging the load from the bowlor secondary classifier into a storage tank, and reusing the stored loadwhen starting up again. Tnis standby can be eliminated under theimproved operation.

From the foregoing it can be seen that the primary classifier accordingto the novel arrangement constitutes what may be called the couplingbetween the primary and the secondary circuit, inasmuch as thatclassifier is closed circuited through the flow of its oversize or rakeproduct with the primary mill, and on the other han'd also closed'circulted through its undersize or overflow product with the secondarymill cycle. Furthermore it can be seen that this classifier not onlyforms a cushion for relative irregularities or fluctuations in thecircuits, but also ofiers a point control of the-system by manipulatingthe operation of the classifier, furnishing means for shifting the bulkof circulating loads back and forth between the stages.

While all these advantages are derived from the relatively simple changein the fiow sheet as explained there is inherent in the new hookup stillanother advantage discernible when the introduction of the new feed isslightly modified. This modification relates to introducing the new feedas at 48 in Figure 2 into the primary classifier feed which according tothis invention comprises the primary plus the'secondary mill discharge.Introducing the new feed which may bedry, into this ample feed volumegives an opportunity to thoroughly wet the same, and furthermoreseparating certain fines directly out of the new feed as it passesthrough the primary classifier, prevents the fines which are containedin some of the raw feeds in excess, from unnecessarily passing throughthe primary mill (into which ordinarily the new feed is firstintroduced).

According to one desirable arrangement, both mills may be located atsubstantially the same elevation, the mixture of both gravitating to apump by which it is lifted through a riser pipe to the primaryclassifier located at a suitable height elevation. The rake product fromthis classifier is allowed to gravitate to the primary mill feed whereit mingles with the new feed. The elevation of the primary classifier ishigh enough, not only to allow unhampered gravitation of the coarse tothe mill feed, but also high enough to allow the classifier overflow tofeed the bowl of the secondary classifier which in turn may dischargeits rake product gravitationally into the secondary mill feed. Since thefeed to the primary classifier, according to the improved operation isof greater volume and contains a relatively larger proportion of fines,it will be therefore more easily and evenly distributed over theclassifier feed trough or inlet (see Figures 2 and 3) than washeretofore possible. This is particularly desirable where the classifieris of considerable width and capacity such as is often called for wherehigh circulating loads are desired; The bowl overflow from the secondaryclassifier may feed a customary thickener (not shown) to dewater thefinal slurry.

The preferred-embodiment also shows the use of a somewhat modifiedcompartment mill unit. A standard compartment mill ordinarily used fortwo stage straight grinding has an arrangement for the separate disposalof each mill discharge, including a partition between both millcompartments, leaving discharge space on each side thereof, separateoutlet means and separate sump and pump arrangements for each discharge.This is now replaced by a single discharge chamber of reduced sizebetween the mills, eliminating the partition with a single sump below,and single pump, making the whole a simple and compact aggregate.

It will be readily seen that with pumping now rendered easy andreliable, the elevations. and.

relative location of the various machine units of the system can be moreliberally chosen; various types of mechanical feeders, elevators and thelike can be eliminated; the mill designer having less restrictions onhim can make better use of, and adapt design more readily to given localconditions, at a reduced expense of layout and machinery involved.

,I claim:

1. A method for closed circuit two stage grinding which comprisescirculating new feed through a primary operating cycle including a wetcoarse grinding stage and a wet classifying stage, returning theclassified coarse product to the coarse grinding stage, advancing theintermediate product separated from said classifying stage into asecondary operating cycle including a wet finegrinding stage and anotherwet classifying stage ahead of' said fine grinding stage, dischargingthe fines separated from the secondary classifying stage forutilization; characterized by the steps of mixing discharge from theprimary and from the secondary grinding stages, and returning themixture to the primary classifying stage.

2. In a closed circuit two stage grinding system, the combination with aprimary operating cycle including a coarse wet grinding mill, and a wetoperating classifier; a. secondary operating cycle including a wet finegrinding mill, and a wet operating classifier; of pump means and conduitconnections therefor, arranged for transferring combined discharges fromboth mills to the primary classifier.

3. A method for closed circuit two stage grinding which comprisescirculating new feed through a. primary operating cycle including a wetcoarse grinding stage and a wet classifying stage, returning theclassified coarse product to the coarse grinding stage, advancing theintermediate product separated from said classifying stage into asecondary operating cycle including a wet fine grinding stage andanother wet classifylng stage ahead of said fine grinding stage,discharging the fines separated from the secondary classifying stage forutilization; characterized by the step of pumping combined dischargefrom the primary and the secondary grinding stage into the primaryclassifying stage.

4. A method for closed circuit two stage grinding which comprisescirculating new feed through a primary operating cycle including a wetcoarse grinding stage and a wet classifying stage, returning theclassified coarse product to the coarse grinding stage, advancing theintermediate product separatedfrom said classifying stage into asecondary operating cycle including a wet fine grinding stage andanother wet classifying stage ahead of said fine grinding stage,discharging the fines separated from the secondary classifying stage forutilizatiorncharacteriaed bythe step of dischargefrom the primary andthe secondary grinding'stageintheprimaryclassifying ing the classifiedcoarse product to the coarse 1o grinding stage, advancing theintermediate prodnot separated from said ciasifying stage into asecondary operating cycle including a wet fine grinding stage andanother wet classifying stage ahead of said fine grinding stage, thefines separated from the secondary classifying stage for utilization;characterised by the fact thatamixtureofdischargefromtheprimary and fromthe secondary grinding stages is treated intheprimaryclassifyingstage.and thatnew feed is introduced together with circulating feed to theprimary classifying stage.

8. A method for closed circuit two stage grinding which comprisescirculating new feed through a primary operating cycle including a wetcoarse grinding stage and a wet classifying stage, returning theclassified coarse product to the coarse grinding stage, advancing theintermediate product separated from said classifying stage into asecondary operating cycle including a wet fine grinding stage andanother wet c stage ahead of said fine grinding stage, discharging thefines separated from the secondary classifying stage for utilization;characterized by the fact that a mixture of discharge from the primaryand fromthesecondarygrindingstagesistreated in the primary classifyingsta e. whereby combined fiow from both grinding stages is pumped to theclassifying stage, and that the intermediate overfiow separation isallowed to gravitate from the primary classifying stage to the secondaryclassifying stage. and the secondary coarse separation to gravitate tothe secondary grinding stage.

'I. A method for closed circuit two stage grinding which comprisescirculating new feed through a primary operating cycle including a wetcoarse grinding stage and a wet classifying stage, returning theclassified coarse product to the coarse grinding stage, advancing theintermediate product separated from said classifying stage into asecondary operating cycle including a wet fine grinding stage andanother wet classifying stage ahead of said fine grinding stage,discharging the fines separated from the secondary classifying stage forutilization: characterized by the fact that a mixture of discharge fromthe primaryand from the secondary grinding stages is treated in theprimary classifying stage.

8. A method for wet grinding in stagewise op eration, in which anoperating stage comprises a step of wet grinding and a step of wetclassifyin said steps to operate in closed circuit relation to eachother with oversize material from the classifying step returning to thegrinding step, delivering a slurry containing undersize material fromsaid classifying step into another similar closed circuit operatingstage and which comprises another wet grinding step and another wetclassifying step, and reintroducing a controlled amount of the materialregrormd in the latter circuit into the c stage of the precedingcircuit.

9. A method for wet grinding in stagewise operation, in which anoperating stage comprises a stepof wet grindingandastepofwetclassifying,said steps to operate in closed circuit relation to 7 each other withoversize material returning to the grinding step, delivering a slurrycontaining undersize material from said classifying step into anothersimilar closed circuit operating stage comprising another wet grindingstep and another wet classifying step, and reintroducing into the firstmentioned circuit an amount of slurry in transit in the latter circuit,containing imdersize produced by the latter grinding step, saidundersi'ze in passing through the first mentioned classifying step beingeffective to change the dilution or'viscosity of the slurry therein andthereby consequently changing the buoyancy of the slurry with respect tocoarser particles, in order to efiect the grade of separation desired insaid first mentioned classifying step.

10. A method for wet grinding in stagewise operation, in which anoperating stage comprises a step of wet grinding and a step of wetclassifying, said steps to operate in closed circuit relation to eachother with classified oversize material returning to the grinding step,delivering a slurry containing undersize material from said classifyingstep into another or later similar closed circuit operating stagecomprising another wet grinding step and another wet classifying step,withdrawing from the process a slurry containing fines produced in thelater circuit and reintroducing it into the earlier circuit, said finesin passing through .the first mentioned classifying step being effectiveto change the dilution or viscosity of the slurry therein and therebyconsequently increasing the buoyancy of the slurry with respect tocoarser particles, in order to aid a desired coarser separation in saidclassifying step.

11. A system for stage closed circuit grinding comprising a wet grindingstage and a wet classifying stage operating in a primary closed circuit,and another wet grinding stage for further comminution of underslzematerial obtained from the overflow of the first-mentioned classifyingstage and another wet classifying stage operating in circuit with saidsecond grinding stage characterized by means for combining milldischarge from both grinding stages, and conveying means for introducingsuch mixture into the primary classifying stage to effect therein aredistribution of the proper and desired sizesolids from the mixtureinto and over the respective grinding circuits.

12. A closed circuit two-stage grinding system according to claim 13 andhaving in said circuits a primary and a secondary classifierrespectively of the settled solids conveying or rating type with a rakeproduct discharge and an overflow, and having correspondingly a primaryand a secondarymill, in which system a pump lifts the combined milldischarges to the primary classifier located at an elevation above themill feed, in which are provided means for allowing gravitationaldischarge of the rake product of the primary classifier to feed therespective primary mill, in which are also provided conduit means forgravitational feed of the primary classifier overfiow to the secondaryclassifier; and in which are provided conduit means for gravitationallyconveying secondary rake product to the feed of the secondary mill.

13. A system of stagewise closed circuit grinding which comprises aprimary and a secondary closed circuit grinding arrangement for coarsegrinding and fine grinding respectively, each such grinding arrangementincluding a wet grinder of the rotary drum type and a wet-classifier ofthe mechanical agitating conveying type, a connection for the primaryclassifier efiective to rehaving interposed therebetween a dischargedelivery space, each section having a feed arrangement for supplyingrespective circulating loads to the respective grinding sections, eachsection also having a discharge arrangement for ground material, meansfor combining the respective materials discharged from said grindingsections, means for returning the combined discharges to the classifierof the primary grinding arrangement, the size separation in said lastmentioned classifier being controllable by controlling the operation ofthe classifier to effect a load distribution with regard to the grindingstages to enable each grinding section to be kept under a desired loadwhile the unit rotates, as by the shifting of load, for instance, fromthe secondary grinding compartment to the primary grinding compartmentof said rotary drum unit.

14. A method of stagewise closed circuit grinding in which the materialpasses through a primary and a secondary cycle, each cycle comprising awet grinding and a wet classifying step coupled in a circuit, in whichmethod discharge from the secondary. grinding step is returned to theprimary classifying step and in which portions of the grinding load areshiftable between a1,osa

said grinding stages, for instance, from the second to the firstgrinding stage, by efi'ecting a controlof size separation in saidprimary classifyin stage.

15. A stagewise wet grinding system comprising a first grindingstage'and a first classifying stage; a second grinding stage, and asecond material from the discharge of both grinding stages to the firstclassifying stage.

16. A method of stagewise wet grinding, which comprises grindingmaterial to be treated in parallel operation in a first and in a secondgrinding stage; introducing new feed material into the system by way ofthe first grinding stage; transferring material from the discharging ofboth grinding stages to a first wet classifying stage; recyclingclassified material from the first classifying stage to the one 01' saidgrinding stages while transferring the overflow from that classifyingstage to a second wet classifying stage; and recycling classifiedmaterial from said second classifying stage to the other of said grindistages.

JAMES V. BLADE.

